The present invention relates to an optical apparatus including a photodetector having a plurality of light receiving sections to receive a condensed light beam, a tracking apparatus including the optical apparatus, and a disk apparatus including the optical apparatus. The present invention particularly relates to an optical apparatus which compensates for offset resulting from displacement of an optical axis of a condensed light beam with respect to a photodetector.
A photodetector for receiving a condensed light beam is generally designed as a two-divided detector or a four-divided detector. The two-divided detector has a light receiving section which is divided into a couple of light receiving regions to receive a condensed spot of a condensed light beam. The four-divided detector has a light receiving section divided into four light receiving regions. The two-divided detector is arranged in such a manner that a spot of a condensed beam equally illuminates both light receiving regions. Moreover, the four-divided detector is arranged in such a manner that a spot of a condensed beam illuminates equally the four light receiving regions.
These two-divided detectors and four-divided detectors are used in an optical disk apparatus. The two-divided detector is typically used as a detector for tracking error detection and as a detector for reading information. The four-divided detector is typically used as a detector for focus error detection.
FIG. 1 shows an optical system of an optical disk apparatus. The optical system shown in FIG. 1 includes a detector for tracking error detection and a detector for focus error detection. The light beam emitted from a light source 7 becomes a coherent light beam through a collimator lens 12. The coherent light beam passes a beam splitter and is then focused by an objective lens 20. The focused light beam illuminates a disk 9. The objective lens 20 forms a light beam spot on the disk 9.
The disk 9 is an optical disk on which many information tracks or other targets are formed. In FIG. 1, the optical system structured by including the collimator lens 12, the beam splitter 13, and the objective lens 20 is called a light focusing optical system 8. The light beam focused on a recording surface of the disk 9 is also reflected by the recording surface. Moreover, the light beam is diffracted depending on the information recorded on the disk 9. The reflected light beam is converted to a parallel beam passing through the objective lens 20. Thereafter, the reflected light beam is bent in its light path by 90 degrees with the beam splitter 13. The light beam of which the light path direction has been changed is then passed into a beam splitter 14.
This coherent light beam is split, by this beam splitter 14, to a light beam directed to the optical system 10 for focus error detection and a light beam directed to the optical system 11 for tracking error detection. The optical system 10 for focus error detection comprises a four-divided photodetector 5, an optical system 10a introducing an asymmetrical spot shape of the condensed light beam, and a condenser lens 10b for condensing the light beam on the four-divided photodetector 5. The optical system 11 for tracking error detection comprises a two-divided photodetector 6 and a condenser lens 11a for condensing the light beam on this two-divided photodetector.
The light beam which is deflected by the beam splitter 13 is divided into one light beam input to a reproducing optical system (not illustrated) for reproducing information recorded on the disk 9, and another light beam entered into the beam splitter 14.
In the optical system 11 for tracking error detection, deviation between the information track formed on the disk 9 and the beam spot formed on the disk 9 by the objective lens 20 is detected. As the detecting method, various methods are known. The most popular method utilizes the diffraction phenomenon generated on the recording surface of the disk 9. In a rewritable magneto-optical disk, a guide groove is formed between adjacent tracks. This groove is arranged as many grooves formed in the equal interval in the track width having constant width. Many grooves enables the disk surface to work as the diffraction grating. When a beam spot having a diameter almost equal to the track width is focused on the disk surface and is then reflected therefrom, the mirror-surface reflected light beam and diffracted light beam are generated. The diffracted light beam interferes with the light element reflected by the mirror-surface to largely change the distribution of light beam intensity reflected from the disk 9. The beam could also be directed through the disk 9 if the disk were transparent.
FIG. 2 shows the distribution of reflected beam intensity influenced by the diffracted light beam. FIG. 2(a) shows the distribution of intensity when the beam spot is located at the center of the track. FIG. 2(b) shows the distribution of strength when the beam spot is located at the position deviated by about xc2xc the width of the track from the center of the track. Distribution of the light beam intensity is largely different at the portions where the mirror-surface reflected light beam and diffracted light beam are overlapped or not overlapped. In this case, the reflected light beam is divided into a left side region and a right side region with a straight line (broken line A in the figure) which passes the center of the reflected light beam and is parallel to the track. The beam intensity in the left side region of the straight line A is compared with that in the right side region. In the case of FIG. 2(a), the beam intensity in the left side region is equal to that in the right side region. Meanwhile, in the case of FIG. 2(b), since the diffracted beam is unbalanced, a certain difference is generated in the beam intensity of the left side and right side regions. This difference in intensity is detected by the two-divided photodetector having the dividing light parallel to the track on the disk 9. Namely, the difference of output of the two-divided photodetector 6 corresponds to the difference of beam strength and also to the amount of deviation of the spot from the center of the track. Therefore, such amount of deviation is detected in higher accuracy. This method is called a push-pull method.
This push-pull method detects the amount of deviation revealed by the difference of strength of the two-divided photodetector. Therefore, when the beam spot center is deviated from the dividing line of the two-divided photodetector, offset is generated in the differential output of the two-divided photodetector.
FIG. 3 shows the condition where deviation of the optical axis occurs. In FIG. 3, a chain line indicates the main beam 4a of the light beam when the optical axis 20a of the objective lens is aligned with the optical axis of the light beam 4. When the objective lens 20 moves upward (direction of arrow mark B) and is located at the position indicated by a broken line, the main beam of the light beam is input to the objective lens 20, but the main beam of the light beam is deviated from the optical axis 20a of the objective lens 20. The light beam is refracted by the objective lens 20 and is then directed to the disk 9. The light beam is reflected by the disk 9 and is then input to the objective lens 20. The optical axis 4b of the main beam of the incident light beam of the objective lens 20 is deviated upward (direction of arrow mark B) from the optical axis 20a of the objective lens 20. This reflected light beam is indicated by a broken line.
Thereafter, the light beam passes the beam splitters 13, 14 and is then input to the optical system 11 for tracking error detection. Next, this light beam is input to the condenser lens 11a at the position where the main beam thereof is deviated downward (direction of arrow mark C) for the optical axis of the condenser lens 11a of the optical system 11 for tracking error detection. The main beam input to the condenser lens 11a is refracted by the condenser lens 11a and is then input after it is deviated downward (direction of arrow mark C) from the photodetector 6.
The entire part of the light beam reflected by the disk 9 is deviated downward of the photodetector 6 around the optical axis 4b of this deviated main beam. As explained above, if the optical axis of the main beam of the light beam is deflected from the dividing line of the two-divided photodetector, deviation is generated between the symmetrical line of the strength distribution and the dividing line of the two-divided photodetector. Therefore, a differential output of the two-divided photodetector includes offset. This differential output is called as a push-pull signal.
FIG. 4 shows a push-pull signal. The horizontal axis of FIG. 4 indicates the position in the direction crossing the track, while the vertical axis of FIG. 4 indicates level of the push-pull signal.
In FIG. 4, the push-pull signal (a) indicates the push-pull signal in such a case that there is no deviation between the optical axis of the objective lens and that of the light beam input to the objective lens, while the push-pull signal (b) indicates the push-pull signal in such a case that there is a deviation of 200 xcexcm between the optical axis of the objective lens and that of the optical beam input to the objective lens. The push-pull signal (b) corresponds to the case where the objective lens is deviated from the aperture diameter by about 5%. The aperture diameter of the objective lens is 3.3 mm. The track width is 1.1 xcexcm.
The push-pull signal (a) corresponds to the push-pull signal where the condensed spot of light beam has moved, with no displacement of the optical axis, to the groove in the opposite side crossing the track from the one groove of the track. If there is no displacement of the optical axis, the push-pull signal becomes zero when the focus spot is located at the center of the track. When an optical head or objective lens 20 is moved to make the push-pull signal zero, the focus spot can be accurately located to the center of the track. Moreover, the signal waveform of the push-pull signal is perfectly symmetrical in the positive and negative sides of the coordinates 0, so it is the ideal control signal.
The push-pull signal (b) corresponds to the push-pull signal as the focus spot of light beam moves to the groove in the opposite side crossing the track from the one groove of the track when there is a displacement of the optical axis. In this case, if the focus spot is located at the center of the track, the push-pull signal does not become zero. When location of the focus spot is controlled to make the push-pull signal zero, the spot is located to the position deviated from the center of the track. The push-pull signal (b) of FIG. 4 includes the deviation of about 0.1 xcexcm as the offset. This displacement of the optical axis generated when the objective lens moves can be generated by assembling errors of the condenser lens.
In general, when information is read from a disk, positional deviation between a focus spot and a track center is generated due to eccentricity of the disk. The positional deviation between the focus spot and the track center can be corrected when the objective lens 20 moves in the direction crossing orthogonal to the track direction. Since such positional deviation is often generated, movement of the objective lens is also generated frequently and thereby optical axis displacement of the optical beam also occurs frequently. If the optical axis center of the objective lens is deviated by the distance d from the optical axis of the light beam, the optical axis of the reflected beam is deviated by 2d from the optical axis of the objective lens.
If the disk 9 deflects the optical axis of the light beam by an angle "THgr", when the focal distance of the objective lens is defined as f, the amount of displacement can be expressed as 2f"THgr". In addition, when the track density on the disk becomes higher, the degree of influence by offset due to the optical axis deviation and degree of influence by assembling errors becomes larger.
An offset of the tracking error signal will interfere with the data read/write operation. A typical optical disk apparatus is provided with an optical disk medium having a recording surface in which the width of the information track is about 1 xcexcm. Data is recorded along this information track. In order to read this data, the focus spot of the light beam must be accurately positioned on this information track. The width of the recording pits formed at the center position in the width direction of the track is narrower than the width of the track.
When a condensed spot is deviated too far from the center of the track during a data reading operation, (i) data can no longer be read because the focus spot does not overlap on the recording mark, or (ii) data reading accuracy and speed are lowered because the data signal level becomes lower than the noise signal level.
When a disk is exchangeable for a disk drive and data is written under the condition that track deviation is generated within the disk drive, the data written in such a disk cannot be read in some cases with different disk drives.
Displacement of the optical axis during a focus spot position adjustment can be avoided by conducting a focus spot position adjustment by moving the entire part of the optical pickup including a condensing optical system and optical system for tracking error detection. In this case, since the optical pickup as a whole, which is heavier than the optical system for tracking error detection, is moved, the maximum response rate of a condensed spot position adjusting operation is limited.
Moreover, an offset included in a push-pull signal because of deviation between an optical axis of an objective lens and that of a light beam may be canceled by adding a bias signal to the push-pull signal depending on a signal indicating the amount of movement of an objective lens from a position detector which detects the position of the objective lens. In this case, the position detector and a bias signal adding circuit must additionally be provided.
In addition, as shown in Japanese Laid-Open Patent Application No. SHO 59-38939, there is a method in which an offset compensating signal is produced from a specific shape of pits formed on the track of the disk. In this case, the specific shape of pits must be formed on the disk and thereby the disk format must be changed.
Moreover, as shown in Japanese Laid-Open Patent Application No. HEI 8-306057, there is a method in which an offset compensating signal can be produced using a particular arithmetic formula from many receiving outputs from the photodetector in which the light receiving section is divided into many regions. In this case, a circuit for conducting the arithmetic operation indicated by the particular arithmetic formula must be provided.
In the methods explained above, offset resulting from the condenser lens used in the optical system for tracking error detection and assembling error of the photodetector cannot be corrected. In addition, the methods explained above are proposed to eliminate offset included in the differential output of the photodetector in the optical system for tracking error detection, and these methods cannot be applied as a method of correcting offset of the photodetector of the other optical system, namely, of the optical system for focus error detection and optical system for information reading.
It is an object of the present invention to provide a new and improved optical apparatus which can eliminate negative effects of offset resulting from assembling errors.
It is another object of the present invention to provide a new and improved optical apparatus which can correct for offset of a light beam or photodetector in any type of optical system.
Still another object of the present invention to provide a new and improved optical apparatus which can easily and economically eliminate the effects of offset without any additional detector and addition of any particular circuit.
Yet another object of the present invention is to provide a new and improved optical apparatus which can move at high speed.
It is a further object of the present invention to provide a new and improved tracking apparatus which can realize a high precision tracking operation.
It is a still further object of the present invention to provide a new and improved optical disk apparatus which can realize a high precision information reading operation.
In keeping with one aspect of the invention, an optical apparatus has a photodetector including a plurality of light receiving sections to receive a light beam condensed by a condenser lens. An optical element is provided for relatively increasing the amount of receiving light of one light receiving section and relatively limiting or reducing the amount of receiving light of another light receiving section, depending on displacement of the optical axis of the light beam.
In another aspect of the invention, a tracking apparatus is provided for tracking the focus spot of light beam on a track or other target on the basis of a tracking error signal obtained from an optical system for tracking error detection. The optical system for tracking error detection includes an optical apparatus having a photodetector which outputs, as a tracking error signal, a difference value of outputs of a plurality of light receiving sections for receiving the light beam condensed by the condenser lens. It also has an optical device for relatively increasing the amount of light received by one light receiving section, which relatively reduces the amount of receiving light of another light receiving section, depending on displacement of the optical axis of the light beam.
Moreover, an optical disk apparatus for reading information from a track while tracking the focus spot of light beam on the track on the basis of a tracking error signal obtained from an optical system for tracking error detection, can include an optical apparatus having a photodetector and an optical element which compensates for off-track conditions.